Screw and nut mechanism for steering gears



Sept. w, 1940. N. TRBOJEVICH 2,214,493

SCREW AND NUT MECHANISM FOR STEERING GEARS Filed April 13, 1939 Patented Sept. 10, 1940 I i v UNITED STATES PATENT OFFICE soar-2w m NUT uscnmsu roa s'raaamo GEARS Nikola Trboieviclr. Detroit, Mich.

Application April 13, 1939, Serial No. 267,666

4 Claims. (Ci. ll-456) z This invention relates to a screw, ball and nut Figures 4, 5 and 6 are geometrical diagrams mechanism which is particularly adapted for explanatory of the calculations found in the deactuating the link mechanism of steering gears, scription. I and for other similar purposes. As is shown in Figures 1 and 2, the screw ii The subject matter is a continuation in part consists of a cylindrical body and two integral 5 of my former invention described in my copendextensions, one at each end of the same, whereby ing application for patent, Serial No. 251,239, the said screw may be rotatably mounted in two flied January 16, 1939. In that application I bearings in a housing. The left end extension advanced the idea of mounting all bails intercomprises a conical race I! upon which a pluvening the nut and screw in a floating sleeve rality of taper rollers l3 held in the outer race l4 10 member equipped with two flanges at a predeand in the housing It, are mounted. At the right termined distance from the corresponding ends end of the screw a larger cylindrical shank it of the nut whereby the position of the sleeve at is formed, the said shank being provided with a the end of each stroke would be corrected by the bore ll into which the steering tube i6, usually motion of the nut thus compensating for the manually operated from the steering wheel, is slippage of the balls and any other deviation fitted. The outer circumference of the said shank from the calculated rolling motion. may be vused for mounting upon a convenient I shall now describe a mechanism in which ball or roller bearing, not shown in the drawing. the said sleeve member is positively guided at The central part of the said screw II is proall times, thus preventing the slippage of the vided with two screw threads arranged side by 94) balls in any position. This continuous correction side, of the same hand but of a different contour and guidance of the balls is accomplished in inand lead. The screw thread I! on the left side flnitesimal steps by means of a differential screw is preferably a square thread or, at least, a and nut in this preferred construction. However, thread so formed that considerable cylindrical the novelty resides in the mechanical principle bearingareas 26 are left uncut at its outer cir-. itself rather than in its any particular embodicumference. At the other side of the gap 2i ment. which forms the dividing line between the two The object of this invention is to constantly threads, the main thread 22 is formed, said thread control the rolling of all balls in order to insure having a hollow circular cross contour correa smooth and safe operation of the mechanism. spending to the balls 23 and also has the above Another object is to reduce the number of mentioned cylindrical bearing areas 26 lying beballs employed. Heretoiore, in mechanisms of tween its grooved portions. this kind some balls were doing the actual work The sleeve 24 is a cylindrical shell smoothly while others were returning through a channel. finished all over in order to provide bearing sur- In this construction all balls are simultaneously faces contacting the uncut areas 20 of the threads active. l9 and 22 with its inner circumference and the Another object is to construct a modification similarly formed contact areas 20 in the nut in which only one row of balls is used, as in a thread 33 with its outer circumference. A plu-. ball bearing. This arrangement renders the rality of smoothly reamed circular perforations mechanism available for low priced vehicles on 25 are further formed in its wall, said perfora- 49 account of its reduced size and cost. tions being aligned along one full convolution of Still another object is to assemble the balls the helix-substantially lying in the central porunder an initial pressure which object is now tion of the broken plane 2-2 and each encircling feasible on account of the limited number of and a ball 23 with an accurate and snug fit. The left the geared control over the said balls. end of the sleeve 24 is formed into a flange 26, 5

In the drawing: the said flange contacting with its two sides, the Figure 1 shows the elevation in cross section inner wall of the shell 21 and the flank of the of the improved mechanism. differential nut 28, respectively. The nut 26 Figure 2 is the cross section of Figure 1 taken meshes with the differential thread I9 and is in the helix 2-2 passing through the ball cen- -bolted at its circumference by means of the ters. screws 29 to the said shell 21. At a certain Figure 3 is a fragmentary and diagrammatic point of its circumference, the said nut is proview of the detail comprising the curved slot guidvided with a guide pin 30 engaging the correing the differential nut pin shown in Figures 1 spending longitudinal slot 3! formed in the wall and 2. of the housing i5 whereby the said nut is prevented from rotating, but it may freely translate in the axial direction of the screw. The nut 23 and the sleeve 24 thus form a single unit in which the two portions are freely rotatable relatively one to the other, but the total length of the two members remains unchanged during any such rotation.

The main nut 32 is a circular shell having a thread 33 formed in its inner circumference throughout-its entire length, the said thread exactly corresponding in its hand, pitch and cross contour to the already described main thread 22. At the outer circumference of the said nut, an outwardly extending journal 34 is formed integrally with the same by means of which the nut may be connected to the steering linkage of the vehicle, preferably by means of spherical bushing mounted upon the journal 34 and a lever connected to the steering cross shaft 35 diagrammatically shown by means of a dotted circle, as was fully described and illustrated in my above cited copending application for patent. If the connection between the nut and the steering shaft be of the indicated kind, the path of the Journal 34 will be a circular arc 36 concentric with the dotted circle 35, Figure 1, and the nut 32 will then rock through an angle b, Figure 2, as it translates throughout the stroke 8. It is now necessary to cause the differential nut 28 to rock in unison wtih the main nut 32, i. e., with the same amplitude b. This is accomplished by forming the groove 3| which guides the pin 30 to conform with a longitudinally disposed curve 31 in such a manner that the two nuts, while translating with different linear velocities, will both rock through the same angle b at any one instant, see Figure 3. It is true that this same movement could be accomplished more directly by connecting the two nuts 28 and 32. by means of a sliding-driving coupling such as a spline, but I prefer the arrangement shown in Figure 1 for certain minor practical reasons, such as the ease of assembling the parts.

The action of the mechanism will be understood from this. When the screw I I is rotated in its bearings with a unit angular velocity, one unit for each complete turn, the main nut 32 will translate with a linear velocity 11 (thus temporarily disregarding the rocking action), where 1.) denotes the pitch of the main screw thread 22. At the same time, the sleeve member 24 containing the balls 23 will rotate with a velocity It and translate with a velocity p (l-k), where k is what I term, the rolling coefficient of the device, the value of which coefllcient is always less than one-half, usually about .4 and which value can be exactly determined by calculation as it will be presently shown.

The geometrical principle underlying this invention consists in constraining the sleeve member to translate with a velocity p (l-lc) relative to the rotary screw ii. If this be done, the sleeve will also simultaneously rotate of necessity with an angular velocity k, because it cannot get out of the main helix by the virtue of the interposed balls. The translation of the sleeve 24 is accomplished by means of the differential nut 28 which being mounted upon the thread I 9 of the exact lead p (1-70) and also being prevented from rotating by means of the guide pin 30 and the slot.

velocity It thus enabling and also compelling, all balls to roll in their grooves.

From this the corresponding lengths of the cooperating members may be determined. In Fig-' ure 1 the nut 32 is shown in its extreme left position and, upon rotating the screw ii clockwise it begins to translate to the right for the stroke S. During this period the differential nut 23 and the sleeve 24 coupled thereto will describe the path S (Z-k)-. The length of the sleeve 24 exceeds the length of the main nut 32 by a distance Sk, for which reason both members will arrive at the end of the stroke at the same time, the nut overtaking the sleeve in the end. The balls 23 which occupy the leading convolution of the nut helix at the beginning of the stroke 8, will gradually fall behind and will occupy the last convolution at the end .of the stroke, the distance between the first and last positions of the balls relative to the nut being equal to Sk.

The rocking motion of the nut does not affect the rolling of the balls providing that the two nuts 32 and 28 respectively are so coupled together that they both rock simultaneously and with the same amplitude. as it will be now explained.

In Figure 4 the absolute path element ds along the curve 36 may be subdivided into a series of infinitesimal translations dz along the worm axis 38 and another series of rotations du perpendicular thereto. During the periods of translation (of the nut) the worm, not being able to translate, must equivalently rotate thus producing a relative sliding. However, during the elementary periods of rotation dy both elements rotate together with the balls and the relative motion between all these members is nil.

I shall now calculate the exact value of ths rolling coefficient k. In Figure 5 the outer circle 32 is stationary while the inner circle ll rotates through the arc DB describing the angle d. During this period the ball 23 rolls without slipping into the position 23a represented with a dotted circle describing the arc DA or the angle kd. This defines the meaning of coeflicient k.

The developed arc lengths AB and AB in the outer and inner circles respectively are equal, from which with reference to the Figure 5, the following three equations may be written down by inspection:

arc DA arc DB are DA=%; arc AB (2) are DB=arc DA'+arc AB (3) From the above three equations the three arc lengths DA, DE and AB can be eliminated, thus leaving the equation It is now necessary to make two distinct corrections in computing the value of k. First, the balls 23 do not, as a rule, contact their helical grooves at the points A and 'D lying diametrically opposite in a line perpendicular to the screw axis but rather, they lie at two intermediate points E and F, as shown in Figure 6. Second, the arc lengths of the two helixes passing through the contact points E and F respectively, are not equally proportional to their corresponding radii because each has a different helix angle.

In particular, in Figure 6 let the contact points It is a be noted that E and F be selected as the points of concentration of the axial thrusts 40 at which points it is without slipping. Upon and II about the axis EG and FH respectively, which are proportional to the lengths of helical arcs passing through the points E and F. or graphically as shown in Figure 6. If EM be the'radius of the helix E, I lay off perpendicular thereto in the axis 39 the distance The hypotenuse EG the arc length of the first helix. The second hypotenuse FH is obtained by a similar construction, as shown. The exact value of the coemcient It will then be: r

FH k=m E. D. (5)

in this mechanism it is not only possible to control the-general motion of the balls, but it also is possible to preselect the helixes E and F along which the balls should roll. This feature may be practically useful for two reasons. First, the points E and F may be preselected as the foci of maximum stress concentration, thus reducing the frictional energy loss to a minimum. Second, in a certain existing ball race, it might be desirable to artificially shift the point of rolling away from the edge of the race.

What I claim as new is: 1. A screw and nut mechanism comprising a rotatable screw having a main screw thread and a differential screw thread of a lesser lead formed at its circumference, a nut engaging the diflerential thread, a cylindrical sleeve member enveloping the screw and containing a plurality of balls rotatably housed in its wall in a plurality of perforations, amain nut having the same lead as the said main thread and capable of contactin This can be done by calculation will 'be then proportional to.

the balls which in turn contact the said main thread, means for synchronizing the rotation, if any, of the differential and the main nuts and means for operatively connecting the differential nut to the sleeve by means of a relatively rotatable but not axially movable connection.

2. A screw and nut mechanism comprising a rotatable screw having an external helical ball groove and a diiferential screw thread of a lesser pitch formed thereupon, a plurality of'balls in the said groove, a tubular sleeve member having a plurality of perforations and contacting the said balls with its side wall in the said perforations, a nut having an internal ball groove and contacting theballs at their surfaces protrudingfrom'the sleeve member, another nut meshing with the diiferential thread and rotatably but not axially movably en aging the said sleeve member and means for synchronizing the rotations of the two nuts.

3. A screw and nut mechanism comprising a screw and a nut having cooperating helical ball grooves, a tubular sleeve intervening the said members and containing a plurality of balls rotatable in the perforations formed in the sidewall of the said sleeve and also in the said two cooperating ball grooves, an auxiliary differential screw and nut mechanismso connected to the main device that the, dliferential screw rotates in a timed relation with the main screw and the differential nut translates with a linear velocity less than that of the main nut and-means for rotatably connecting the said differential nut to the sleeve member whereby the said sleeve is constrained to translate in unison with the said differential nut.

4. A screw and nut mechanism comprising a cooperatingscrew and nut each having a helical ball groove comprising a plurality of convolutions, a plurality of rolling balls in the said grooves, the said balls forming fewer helical convolutions than the said nut and screw, vening tubular sleeve member enveloping the screw and enveloping the said balls substantially at their mid portions and means for translating the said sleeve in the direction of the relative translation of the said nut with a reduced velocity and in a timed relation.

NKOLA TRBOJEVICH.

an inter- 

